Question: Which of these numbers is composite? ${2,\ 23,\ 31,\ 33,\ 37}$
Solution: A composite number is a number that has more than two factors (including 1 and itself). 2, 23, 31, and 37 each have only two factors. The factors of 33 are 1, 3, 11, and 33. Thus, 33 is the composite number.